Prostate cancer is the most common cancer in men with an estimated 244,000 cases in 1995 in the United States. It is the second leading cause among men who die from neoplasia with an estimated 44,000 deaths per year. Prompt detection and treatment is needed to limit mortality caused by prostate cancer.
As described in W. J. Catalona, “Management of Cancer of the Prostate,” New Engl. J. Med. 331(15):996–1004 (1994) (which is hereby incorporated by reference), the management of prostate cancer can be achieved by watchful waiting, curative treatment, and palliation. For men with a life expectancy of less than 10 years, watchful waiting is appropriate where low-grade, low-stage prostate cancer is discovered at the time of a partial prostatectomy for benign hyperplasia. Such cancers rarely progress during the first five years after detection. On the other hand, for younger men, curative treatment is often more appropriate.
Where prostate cancer is localized and the patient's life expectancy is 10 years or more, radical prostatectomy offers the best chance for eradication of the disease. Historically, the drawback of this procedure is that most cancers had spread beyond the bounds of the operation by the time they were detected. However, the use of prostate-specific antigen testing has permitted early detection of prostate cancer. As a result, surgery is less expensive with fewer complications. Patients with bulky, high-grade tumors are less likely to be successfully treated by radical prostatectomy.
After surgery, if there are detectable serum prostate-specific antigen concentrations, persistent cancer is indicated. In many cases, prostate-specific antigen concentrations can be reduced by radiation treatment. However, this concentration often increases again within two years.
Radiation therapy has also been widely used as an alternative to radical prostatectomy. Patients generally treated by radiation therapy are those who are older and less healthy and those with higher-grade, more clinically advanced tumors. Particularly preferred procedures are external-beam therapy which involves three dimensional, conformal radiation therapy where the field of radiation is designed to conform to the volume of tissue treated, and interstitial-radiation therapy where seeds of radioactive compounds are implanted using ultrasound guidance.
Cytotoxic chemotherapy is largely ineffective in treating prostate cancer. Its toxicity makes such therapy unsuitable for elderly patients. In addition, prostate cancer is relatively resistant to cytotoxic agents.
Androgens are known to stimulate growth of the prostate and other peripheral tissues, including primary or metastatic prostate tumor cells. Testosterone (“T”) is the principal androgen secreted by the testes and is the primary circulating androgen found in the plasma of males. The testes produce 95% of circulating plasma T, while the remaining 5% is derived from the adrenals. In many target tissues, T is converted by the enzyme 5α-reductase to the more potent androgen dihydrotestosterone (“DHT”). T and DHT then compete for binding to the androgen receptor (“AR”) to exert their influence on cell function. DHT has a four- to five-fold higher AR binding affinity than does T and thus serves as the intracellular mediator for most actions of the hormone. However, both androgens contribute to the overall androgenic effect. The high response rate (i.e., 60 to 80%) to first line hormonal therapy and the presence of AR in both primary and metastatic prostate tumor cells support the idea that the AR is an important mediator of prostate cancer development and growth. Denis et al., “Prostatic Cancer: An Overview,” Acta. Oncol., 29:665–677 (1990); McConnell, “Physiologic Basis of Endocrine Therapy for Prostatic Cancer,” Urol. Clin. N. Am., 18(1):1–13 (1991) (which are hereby incorporated by reference)
Because prostatic carcinomas are androgen dependent, various treatment strategies focus on negating the role of androgens (i.e., testosterone and dihydrotestosterone) in prostate tumor growth. Labrie, “Endocrine Therapy for Prostate Cancer,” Endocrinol. Metab. Clin. N. Am., 20(4):845–872 (1991); Soloway, et al., “Antiandrogenic Agents as Monotherapy in Advanced Prostatic,” Cancer, 71(Suppl. 3): 1083–1088 (1993) (which are hereby incorporated by reference). These treatment strategies include use of luteinizing hormone-releasing hormone (“LHRH”) to suppress testicular androgen production or orchiectomy (surgical castration) to eliminate androgen production.
In recent years, antiandrogens have been widely used for the treatment of prostate cancer. The biologic activity of androgens is mediated through the formation of a non-covalent androgen receptor-steroid complex. Antiandrogens inhibit formation if this complex and, thus, negate the role of endogenous steroids in androgen-dependent growth of the prostate.
Antiandrogens can be divided into two groups: steroidal and non-steroidal. There are several non-steroidal antiandrogens such as Flutamide (Eulexin), Nilutamide (Anandrone), and Casodex. All of them bind reversibly to the AR. Accordingly, these antiandrogens can be displayed by an endogenous ligand such as dihydrotestosterone. Therefore, these antiandrogens have not been successful in the treatment of prostate cancer. Earlier reports suggested that dihydrotestosterone bromoacetate (DHT-BA) binds irreversibly to the AR. However, recent evidence showed that DHT-BA binds aldehyde dehydrogenase, and not the AR. McCammon et al., “An Androgenic Affinity Ligand Covalently Binds to Cytosolic Aldehyde Dehydrogenase from Human Genital Skin Fibroplast,” Mol. Cell. Endocrinology, 91:177–183 (1993), which is hereby incorporated by reference. Therefore, DHT-BA is not suitable for treating prostate cancer.
The present invention is directed to overcoming these deficiencies.